1. Field of the Invention
The present invention generally relates to a power transmission and, more particularly, to a manual power transmission for varying the gear ratio between the automotive engine and drive wheels.
2. Description of the Prior Art
A manual power transmission though orthodox it may be nowadays has again gained popularity because of fun of car maneuverability. Of various manual power transmissions available, the power transmission is well known which comprises an input shaft drivingly coupled with the automotive engine, an output shaft drivingly coupled with the drive wheels and coaxially aligned with the input shaft, a countershaft or layshaft disposed parallel to those shafts, and a plurality of speed gear pairs including gears on the input and output shaft and associated countergears on the countershaft.
The power transmission of the structure described above is available in two types. One type is known as an input reduction gear type in which the drive of the input shaft is transmitted to the countershaft with the rotational speed (rpm) of the input shaft having been adjusted at a predetermined gear ratio determined by mutually meshed speed gears on the input shaft and the countershaft, respectively, and is in turn transmitted from the countershaft to the output shaft at another predetermined gear ratio which varies with the selected gearshift position. Briefly speaking, the input reduction gear type is characterized in that reduction in rotational speed takes place between the input shaft and the countershaft.
In this known input reduction gear type, since the speed gears are fixedly mounted on the input shaft and the countershaft, respectively, problems such as, for example, discussed below have been encountered.
(1) Since as a result of reduction in rotational speed a relatively high torque acts on the countershaft, the various gears employed and the input shaft and countershaft must have a high physical strength and a high rigidity such as, for example, by employing the gears of a relatively large face width and the input shaft and countershaft of an increased diameter. This hampers reduction in size and weight of the power transmission, making it difficult to provide a compact power transmission.
(2) When the automotive engine is driven while the automotive vehicle is held parked, that is, during an idle engine operating condition, all of the idler gears are driven idle relative to the countershaft and, therefore, gear clashing tends to occur considerably when rotation of the engine fluctuates.
(3) Since the countershaft is driven at a relatively high torque as a result of reduction in rotational speed accomplished by the speed gear pair and the torque exerted by the countershaft is subsequently increased depending on the gear ratio selected according to the gearshifting, the inertia acting on a gear change mechanism for each gearshift position including, for example, a speed synchronizer tends to become large, making it difficult to reduce the required gearshifting force.
A different type of the power transmission, known as an output reduction gear type, in which reduction in rotational speed takes place between the countershaft and the output shaft, appears to be an effective means for substantially solving the problems inherent in the input reduction gear type. An example of this output reduction gear type is disclosed in, for example, the published European Patent Application No. 0 219 240-A1, published Apr. 22, 1987. The output reduction gear type disclosed in this European publication comprises an input shaft, a countershaft, an output shaft, speed gears including forward drive gears on the input shaft and output shaft, countergears on the countershaft, and means for changing the gears through which power transmission is effected so as to change the transmission ratios. The speed gears on the input shaft are permanently in mesh with, and also in driving connection with, the countergears on the countershaft and are all mounted to rotate freely on the input shaft. Speed synchronizers are used to clutch the speed gears on the input shaft selectively to the input shaft.
In this known output reduction gear type, since the torque of the input shaft is transmitted to the countershaft without being increased, an input load acting on the various speed gears can be minimized. Also, since no fixed speed gear pair intervene between the input shaft and the countershaft, no associated speed synchronizer will be affected by the gear ratio represented by the speed gear pairs and, therefore, the inertia acting on the speed synchronizers is advantageously minimized.
In addition, according to the European publication referred to above, all of the speed synchronizers are disposed around the input shaft and, therefore, the all of the speed gears on the input shaft are rotatable independent of the input shaft, only the input shaft is driven during an engine idling condition, that is, when the power transmission is set to a neutral position with none of the gears on the input shaft being driven. This is effective to avoid clashing of the gears during the engine idling condition.
As discussed above, with the output reduction gear type, the problem inherent in the conventional input reduction gear type discussed earlier can advantageously be eliminated substantially. However, even the output reduction gear type has their own problems or some structural features that require improvement. By way of example, the output reduction gear type has problems associated with increase in rotational speed (rpm) of the countershaft which will now be discussed in detail.
In the case of the conventional input reduction gear type, the rotational speed of the countershaft is reduced according to the gear ratio of the speed gear pair intervening between the input shaft and the countershaft and such gear ratio is substantially fixed regardless of the gearshift position. In other words, the rotational speed of the countershaft is lower than the rotational speed (rpm) of the input shaft regardless of the gearshift position. However, in the case of the conventional output reduction gear type, the rotational speed of the countershaft is subject to variation according to the gear ratio of the gear pair selected according to the gearshift position and will, at a certain high-speed gearshift position or positions, attain a value higher than the rotational speed of the input shaft. Thus, for a given gear ratio, the rotational speed of the countershaft is considerably higher at a certain high-speed gearshift position or positions in the output reduction gear type than that in the input reduction gear type.
As discussed above, the output reduction gear type has the countershaft that tends to be driven at a higher speed than the input shaft. This feature appears to have brought about the following problems.
In the output reduction gear type, the rotational speed of the countershaft is determined by the product of the engine rotational speed times the gear ratio of a selected gear pair and, therefore, as compared with the rotational speed of the input shaft which varies solely with the engine rotational speed and regardless of the selected gear ratio, the range over which the rotational speed of the countershaft changes for all available gearshift positions is substantial. This involves, inter alia, a susceptibility of the countershaft to radial fluctuation as compared with the input shaft, with the consequence that a relatively large load may act on bearings supporting the countershaft.
On the other hand, the speed gears on the input shaft and countershaft are generally in the form of a helical gear and, accordingly, during power transmission from the input gear to the countershaft, a thrust force substantially proportional to the driving torque acts on the input shaft. Considering that the driving torque varies depending on the selected gear ratio, the thrust force acting on the input shaft correspondingly varies with the selected gear ratio, and this is particularly true in the output reduction gear type. This is in contrast to the input reduction type wherein the thrust force generated by the speed gears on the input shaft and countershaft is of a primary concern an the range of change in thrust force is relatively smaller than that in the output reduction gear type. Accordingly, where the lowest available gear ratio in the output reduction gear type is higher than that in the input reduction type, there is the possibility that for a given driving force, a given helical angle and a given tooth-to-tooth engagement between the speed gears on the input shaft and countershaft, the thrust force acting on the input shaft employed in the output reduction gear type is higher than that in the input speed gear.
Although this problem may be alleviated if relatively bulky and robust bearings are employed to increase the support rigidity, the use of the bulky bearings makes it difficult to provide a compact power transmission of the output reduction gear type.
However, the Japanese Laid-open Patent Publication No. 2-93151, published Apr. 3, 1990 (corresponding to U.S. Pat. No. 5,014,567, issued May 14, 1991) appears to have suggested a shaft support system in the output reduction gear type. According to this known shaft support system, ball bearings and roller bearings are employed according to positions where the input shaft and countershaft are to be supported. Specifically, for a given position where the input shaft and the countershaft are to be supported, bearings identical in type and size with each other are employed and, for this reason, it is clear that this known shaft support system has been devised with no regard paid to the difference in range of change of the rotational speed and, also, the difference in range of change of the thrust force.
There is another problem associated with the support of the countershaft. As discussed hereinbefore, the countershaft employed in the output reduction gear type tends to be driven at a higher speed than the input shaft. This involves another problem associated with the support of the countershaft. Considering that in the output reduction gear type the driving torque inputted from the input shaft is reduced according to a selected gear ratio accomplished by a selected speed gear on the input shaft and a mating speed gear on the countershaft, a relatively large torque tends to act on those speed gears giving the selected gear ratio to such an extent as to result in a tendency of the tooth-to-tooth engagement to vary. In the event that the speed gear pair giving the selected gear ratio displaces even the slightest distance in a radial or axial direction, clashing and/or friction sounds may be generated by the gears, accompanied by an eventually accelerated wear of the gears.
According to, for example, Proc. Insts. Mech. Engrs, 1974, Vol. 188 12/74 (pp 169-187), there is disclosed an output reduction gear type in which each of a main shaft, comprised of an input shaft and an output shaft, and a countershaft is rotatably supported at a portion adjacent an input end thereof and input and output ends of the speed gear pair. Specifically, the input end portion of the main shaft and the input end portion of the speed gear pairs are supported by a tapered roller bearing whereas the output end portion of the speed gear pair is rotatably supported by a roller bearing.
Although with the shaft support system disclosed in the literature referred to above the support rigidity can be appreciated to a certain extent since the opposite end portions of the speed gear pair are supported, it is a transmission gear which can be securely retained in position by the tapered roller bearing in an axial direction and the position of the speed gear pair in the axial direction cannot be retained sufficiently.
A further problem with the output reduction gear type is found when the speed synchronizers are disposed around the countershaft. More specifically, considering that in the output reduction gear type, the countershaft generally positioned below the input and output shafts, the countershaft is generally immersed in a quantity of lubricant oil accommodated within the transmission casing. This means that depending on the physical condition of the lubricant oil, the magnitude of a force necessary to accomplish gearshifting, that is, necessary for a driver to apply to a gearshift lever tends to vary. This will be discussed more in detail.
As hereinbefore discussed, the countershaft employed in the output reduction gear type is driven about its own longitudinal axis at a low rotational speed in a low speed drive, but at a high rotational speed in a high speed drive. Accordingly, the lubricant oil in which the countershaft is immersed receives a varying shearing force. The higher the rotational speed of the countershaft, the higher the shearing force acting on the countershaft, and vice versa. The shearing force acting on the lubricant oil does in turn bring about increase of the temperature of the lubricant oil, the gradient of which varies with the magnitude of the shearing force. In general the temperature of the lubricant oil is relatively high at the high speed drive, but relatively low at the low speed drive. The viscosity of the lubricant oil within the transmission casing which is affected by temperature is low at the highest speed gear position and high at the lowest speed gear position.
By the reason discussed above, mounting of the speed synchronizers around the countershaft in the output reduction gear type requires a relatively large gearshifting force at the low speed drive because of the lubricant oil exhibiting a low viscosity, but a relatively small gearshifting force at the high speed drive because of the lubricant oil exhibiting a high viscosity. Thus, with the known output reduction gear type in which the speed synchronizers are mounted around the countershaft, the gearshifting force necessary to accomplish a selected gear ratio varies considerably with position of the gearshift lever.
Furthermore, if in the output reduction gear type in which the rotational speed of the countershaft becomes considerably higher than that in the input reduction gear type as discussed above, all of the speed synchronizers are disposed around the input shaft such as suggested in the European publication referred to above, all of the gears on the input shaft must be the idler gears, that is, rotatable independent of the input shaft and, moreover, one of the gears of each of the firstspeed gear pair and the reverse-drive gear pair, both exhibiting a high gear ratio, which is smaller in gear size than the other, has to be an idler gear on the input shaft.
Accordingly, as will be discussed in detail later, at the high-speed gearshift position or positions, the respective idler gears of those gear pairs will be driven idle relative to the input shaft at a considerably high velocity which may lead to the possibility of associated bearings for those idler gears being undesirably seized. In particular, the idler gear forming a part of the reverse-drive gear pair, which is driven by a reverse idler gears in a direction counter to the direction in which any of the other idler gears on the input shaft is driven, the difference in rotational speed relative to the input shaft tends to increase considerably.
Although not specifically intended to tackle the problems inherent in the conventional output reduction gear type, the Japanese Laid-open Patent Publication No. 2-93151, published Apr. 3, 1990 (corresponding to U.S. Pat. No. 5,014,567, issued May 14, 1991), for example, suggests another output reduction gear type in which the speed synchronizers except for the 3-4 speed synchronizer are disposed around the countershaft and also in which the reverse-drive gear pair is positioned closest to an input-side end wall of the transmission housing. According to this arrangement of the speed synchronizers, one of the gears of each of the first-speed gear pair and the reverse-drive gear pair, both exhibiting a high gear ratio, which is smaller in gear size than the other, is a fixed gear rotatable together with the input shaft and, therefore, the problem associated with the bearing seizure as discussed above can effectively be avoided.
As is well known to those skilled in the art, in the power transmission regardless of the type, supply of lubricant oil to respective meshing regions of the gears of the various gear pairs, the associated bearings for the idler speed gears and/or any other portions that require oiling is generally carried out by stirring the lubricant oil, accommodated in a bottom region of the transmission housing, upwardly by means of some of the countergears on the counter shaft then driven together with the latter. Most of the lubricant oil so stirred upwardly collides against the interior wall of the transmission casing and subsequently falls by gravity along the interior wall of the transmission casing and is cooled in contact with the interior wall of the transmission casing before it is again supplied to the meshing regions, bearings and/or portions that require oiling.
On the other hand, when the automotive vehicle is abruptly accelerated or decelerated or runs along the slope, the level of the lubricant oil accommodated in the transmission casing inclines relative to the transmission casing to such an extent that one of the countergears mounted adjacent an extremity of the countershaft will no longer be able to stir the lubricant oil upwardly, but the lubricant oil will be stirred up by some of the countergears mounted on a generally intermediate portion of the countershaft. In such case, if such some of the countergears on the generally intermediate portion of the countershaft have a relatively small diameter, the amount of the lubricant oil stirred up will become short of the requirement and all necessary portions where require oiling will not be oiled sufficiently.
Considering that in the case of the output reduction gear type, the rotational speed of the countershaft at a certain high-speed gearshift position or positions attains a value higher than the rotational speed of the input shaft as hereinbefore discussed, the gear layout in which the lubricant oil can be stirred up by some of the countergears during the rotation of the countershaft brings about such a problem that the resistance of stirring of the lubricant oil tends to be increased with increase in rotational speed of the countershaft, accompanied by a considerable increase in temperature of the lubricant oil as compared with that in the input reduction gear type to eventually result in difficulty in maintaining a favorable lubrication.
This problem may be substantially eliminated if the amount of the lubricant oil within the transmission casing is reduced to lower the oil level at the bottom of the transmission casing to thereby reduce the stirring resistance, but the lowering of the oil level does in turn bring a different, but detrimental problem in that the amount of the lubricant oil stirred up by some of the countergears during the rotation of the countershaft tends to become short of the requirement. These problems are considerably pronounced in the prior art output reduction gear type such as disclosed in the US patent referred to above since the reverse-drive gear pair including a reverse-drive countergear of a radial size larger than any other countergears is positioned closest to the input-side end wall of the transmission housing.